The research proposed in this application is based on the recent discovery in the applicant's laboratory that a JAZF1-JJAZ1 gene fusion in human endometrial stromal tumors mimics the results of trans-splicing between pre-mRNAs for these two genes in normal endometrial stromal cells. Additional evidence suggests that in the normal endometrium trans-splicing between the pre- mRNAs is physiologically regulated, but in the tumors the gene fusion produces chimeric RNA in an abnormal, dysregulated manner. These data have led to the hypothesis that fusions of genes in other tumors may contribute to the neoplastic phenotype of those tumors in a similar fashion, through the sustained expression of chimeric RNAs normally generated by trans-splicing during specific intervals in cell and tissue development. The preliminary findings and the above hypothesis will be pursued through five specific aims. The first two of these aims (determination of the intranuclear localization of JAZF1 and JJAZ1 pre-mRNAs during the trans-splicing process and identification of an RNA Y-form intermediate that should be unique for trans-splicing) will add further proof for trans-splicing by addressing features of the mechanism for this process. These aims will also provide research procedures that will be useful in studying other examples of trans-splicing. The third aim (analysis of trans-splicing between JAZF1 and PHF1 pre-mRNAs) will investigate a likely second case of a neoplastic gene fusion mimicking trans-splicing in endometrial stromal cells. The fourth aim (analysis of chimeric RNA function in endometrial stromal cells) will address the role of trans-splicing in normal endometrial stroma, and the fifth aim (demonstration of trans-splicing in cells other those of endometrial stroma) will determine whether several, selected gene fusions associated with certain human tumors reflect trans-splicing in the normal tissue counterparts of those tumors. In aggregate, the research proposed in this application will broaden knowledge of RNA trans-splicing in normal tissues and provide new insights into how an important class of DNA rearrangements exerts oncogenic effects in human tumors.